FIGS. 1A and 1B illustrate the configuration described in Japanese Patent Application Laid Open No. 2015-219273 as a related art example of this type of optical module. In this example, the optical module includes a first optical block 11 and a second optical block 12. The first optical block 11 is fixed on a substrate 14 on which one or more optical elements 13 are mounted. The second optical block 12 holding one or more optical fibers 15 is mounted on the first optical block 11. The number of the optical fibers 15 is generally equal to the number of the optical elements 13, and the number of the optical elements 13 is usually greater than or equal to 2.
In a state that the second optical block 12 and the first optical block 11 are interlocked, the optical elements 13 are positioned in a space 11a which is formed on a bottom surface of the first optical block 11. Lenses 11b, the number of which is equal to the number of the optical elements 13, are formed on the first optical block 11. One of the lenses 11b is positioned just above a corresponding one of the optical elements 13 in the state that the second optical block 12 and the first optical block 11 are interlocked.
The second optical block 12 includes four spring pieces 12a. The four spring pieces 12a are formed in a manner to be integrated with a main body part of the second optical block 12. A lance part 12b is formed on an end of each of the four spring pieces 12a. The second optical block 12 is pushed to the top of the first optical block 11 to fit with the first optical block 11. At this time, the lance parts 12b are hooked on an upper end of a step part 11c formed on the first optical block 11 and thus, the second optical block 12 is fixed on the first optical block 11.
A convex part 12c is formed on a bottom surface of the second optical block 12. The convex part 12c is fitted on a concave part 11d formed on an upper surface of the first optical block 11. Lenses 12d, the number of which is equal to the number of the optical elements 13, are formed on the convex part 12c. In the state that the second optical block 12 and the first optical block 11 are interlocked, an optical axis of each of the lenses 11b is accorded with an optical axis of a corresponding one of the lenses 12d. A reflection surface 12e is formed on an upper surface of the second optical block 12. The reflection surface 12e is positioned just above the lenses 12d. 
The optical elements 13 are light emitting elements or light receiving elements, for example. In the case where the optical elements 13 are light emitting elements, each of the lenses 11b converts light from a corresponding one of the optical elements 13 into parallel light. Each of the lenses 12d collects the parallel light from a corresponding one of the lenses 11b. The reflection surface 12e converts travelling directions of all the light from the lenses 12d by 90°. Each of the light from the reflection surface 12e is incident on a corresponding one of the optical fibers 15. The optical fibers 15 are held by the second optical block 12. The extending directions of the optical fibers 15 are parallel to the substrate 14.
Thus, the optical module illustrated in FIGS. 1A and 1B optically connects the optical elements 13 with the optical fibers 15. In this example, an optical connection direction between the first optical block 11 and the second optical block 12 is orthogonal to a plate surface of the substrate 14. A mechanical connection direction (assembling direction) between the first optical block 11 and the second optical block 12 is also orthogonal to the plate surface of the substrate 14.
FIGS. 2A and 2B illustrate the configuration of an optical module described in Japanese Patent Application Laid Open No. 2013-140211. This optical module includes a circuit substrate 22, a housing 24, an optical fiber holding member 26, and an optical coupling member 27, in this example. Optical elements 21 are mounted on the circuit substrate 22. The housing 24 to which an end part of an optical cable 23 is fixed accommodates the circuit substrate 22. The optical fiber holding member 26 holds optical fibers 25 guided from the optical cable 23 into the housing 24. The optical coupling member 27 is fixed to the circuit substrate 22 and optically connects the optical elements 21 with the optical fibers 25. The optical fiber holding member 26 is coupled to the optical coupling member 27 by fitting a guide pin 27a into a guide hole 26a. 
The optical coupling member 27 covers the optical elements 21. On a surface, which faces to the optical fiber holding member 26, of the optical coupling member 27, lenses 27b are formed. On a surface, which faces to the optical elements 21, of the optical coupling member 27, lenses (not shown) are formed. Further, a reflection surface 27c is formed on an upper surface of the optical coupling member 27.
In the case where the optical elements 21 are light emitting elements, rays of light from the light emitting elements pass through the lenses, which are not shown, to be incident on the optical coupling member 27. The reflection surface 27c reflects the rays of light incident on the optical coupling member 27. The lenses 27b optically couple the rays of light from the reflection surface 27c onto end surfaces of the optical fibers 25.
Thus, the optical module illustrated in FIGS. 2A and 2B optically connects the optical elements 21 with the optical fibers 25. In this example, an optical connection direction between the optical fiber holding member 26 and the optical coupling member 27 is parallel to a plate surface of the circuit substrate 22. A mechanical connection direction between the optical fiber holding member 26 and the optical coupling member 27 is also parallel to the plate surface of the circuit substrate 22.
As mentioned above, in the optical module illustrated in FIGS. 1A and 1B, the mechanical connection direction between the first optical block 11 and the second optical block 12 is orthogonal to the plate surface of the substrate 14. Accordingly, the optical module can be easily assembled by pushing the second optical block 12 to the top of the first optical block 11 which is mounted on the substrate 14.
However, the optical module illustrated in FIGS. 1A and 1B has the configuration in which the lance parts 12b are hooked on the upper end of the step part 11c and thus, there is a play in the assembling direction. Therefore, high positioning accuracy cannot be obtained in the assembling direction. In order to permit misregistration in the assembling direction, the optical connection direction of the optical module illustrated in FIGS. 1A and 1B is orthogonal to the plate surface of the substrate 14.
Therefore, it is necessary to form the reflection surface 12e on the upper surface of the second optical block 12. Accordingly, right above the optical elements 13, the first optical block 11 and a part, on which the reflection surface 12e is formed, of the second optical block 12 are overlapped in a direction orthogonal to the plate surface of the substrate 14. Thus, it is difficult to configure a low-profile optical module.
In the optical module illustrated in FIGS. 2A and 2B, since each of the optical connection direction and the mechanical connection direction is parallel to the plate surface of the circuit substrate 22, the reflection surface 27c is formed on the upper surface of the optical coupling member 27 which covers the optical elements 21. Thus, the optical fiber holding member 26 and the optical coupling member 27 are not overlapped with each other right above the optical elements 21, enabling configuration of a low-profile optical module.
However, assembling of the optical module illustrated in FIGS. 2A and 2B is difficult. It is necessary to slide the optical fiber holding member 26 in an extending direction of the optical fibers 25 (that is, in a direction parallel to the plate surface of the circuit substrate 22) in assembling. Therefore, in the case where the optical fibers 25 are fixed at the end part of the optical cable 23, the optical fibers 25 need to be bent. The optical fibers 25 may be damaged due to the bending. Thus, it is impossible to easily assemble the optical module illustrated in FIGS. 2A and 2B.